Nowadays, vehicle drivers are often able to monitor visually the cooling water, gasoline and oil supply, driving and brake lights, indicators, and brake operation, and can be assured of their condition thereby. However, there is still no method of monitoring a tire's internal air pressure by immediate visual inspection, except, of course, when the tire becomes substantially deflated and, as a consequence, deformed and, possibly, damaged. It is well known that, if a vehicle has one tire which is not inflated at its correct air pressure, then the vehicle will run in an unbalanced manner, turn and corner awkwardly, experience abnormal tire and steering wear and, probably most importantly, will tend to brake unevenly and, under certain conditions, lose control, thereby jeopardising safety.